Hot-wall Metalorganic Chemical Vapor Deposition Research Articles

Electrical properties of unintentionally doped β-Ga2O3 (010) thin films grown by a low-pressure hot-wall metalorganic chemical vapor deposition

We investigated the electrical properties of unintentionally doped (UID) Ga2O3 (010) layers grown by low-pressure hot-wall metalorganic chemical vapor depositions from device characteristics of Schottky barrier diodes (SBDs) fabricated on them. Highly resistive properties of the UID Ga2O3 layers were confirmed from current–voltage characteristics. The specific on-resistance of the SBD with the most resistive UID Ga2O3 layer was 2.2 × 107 Ωcm2. Capacitance–voltage characteristics revealed that most of the SBDs had complete depletion of the UID layers at thermal equilibrium, indicating that their residual effective donor densities were less than 3.0 × 1013 cm−3.

High crystalline quality homoepitaxial Si-doped β-Ga2O3(010) layers with reduced structural anisotropy grown by hot-wall MOCVD

A new growth approach, based on the hot-wall metalorganic chemical vapor deposition concept, is developed for high-quality homoepitaxial growth of Si-doped single-crystalline β-Ga2O3 layers on (010)-oriented native substrates. Substrate annealing in argon atmosphere for 1 min at temperatures below 600 °C is proposed for the formation of epi-ready surfaces as a cost-effective alternative to the traditionally employed annealing process in oxygen-containing atmosphere with a time duration of 1 h at about 1000 °C. It is shown that the on-axis rocking curve widths exhibit anisotropic dependence on the azimuth angle with minima for in-plane direction parallel to the [001] and maximum for the [100] for both substrate and layer. The homoepitaxial layers are demonstrated to have excellent structural properties with a β-Ga2O3(020) rocking curve full-widths at half-maximum as low as 11 arc sec, which is lower than the corresponding one for the substrates (19 arc sec), even for highly Si-doped (low 1019 cm−3 range) layers. Furthermore, the structural anisotropy in the layer is substantially reduced with respect to the substrate. Very smooth surface morphology of the epilayers with a root mean square roughness value of 0.6 nm over a 5 × 5 μm2 area is achieved along with a high electron mobility of 69 cm2 V−1 s−1 at a free carrier concentration n=1.9×1019 cm−3. These values compare well with state-of-the-art parameters reported in the literature for β-Ga2O3(010) homoepitaxial layers with respective Si doping levels. Thermal conductivity of 17.4 Wm−1K−1 is determined along the [010] direction for the homoepitaxial layers at 300 K, which approaches the respective value of bulk crystal (20.6 Wm−1K−1). This result is explained by a weak boundary effect and a low dislocation density in the homoepitaxial layers.

Low Al-content n-type Al[formula omitted]Ga[formula omitted]N layers with a high-electron-mobility grown by hot-wall metalorganic chemical vapor deposition

In this work, we demonstrate the capability of the hot-wall metalorganic chemical vapor deposition to deliver high-quality n-AlxGa1−xN (x = 0–0.12, [Si] = 1×1017 cm−3) epitaxial layers on 4H-SiC(0001). All layers are crack-free, with a very small root mean square roughness (0.13–0.25 nm), homogeneous distribution of Al over film thickness and a very low unintentional incorporation of oxygen at the detection limit of 5×1015 cm−3 and carbon of 2×1016 cm−3. Edge type dislocations in the layers gradually increase with increasing Al content while screw dislocations only raise for x above 0.077. The room temperature electron mobility of the n-AlxGa1−xN remain in the range of 400–470 cm2/(V.s) for Al contents between 0.05 and 0.077 resulting in comparable or higher Baliga figure of merit with respect to GaN, and hence demonstrating their suitability for implementation as drift layers in power device applications. Further increase in Al content is found to result in significant deterioration of the electrical properties.

Tuning composition in graded AlGaN channel HEMTs toward improved linearity for low-noise radio-frequency amplifiers

Compositionally graded channel AlGaN/GaN high electron mobility transistors (HEMTs) offer a promising route to improve device linearity, which is necessary for low-noise radio-frequency amplifiers. In this work, we demonstrate different grading profiles of a 10-nm-thick AlxGa1−xN channel from x = 0 to x = 0.1 using hot-wall metal-organic chemical vapor deposition (MOCVD). The growth process is developed by optimizing the channel grading and the channel-to-barrier transition. For this purpose, the Al-profiles and the interface sharpness, as determined from scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, are correlated with specific MOCVD process parameters. The results are linked to the channel properties (electron density, electron mobility, and sheet resistance) obtained by contactless Hall and terahertz optical Hall effect measurements coupled with simulations from solving self-consistently Poisson and Schrödinger equations. The impact of incorporating a thin AlN interlayer between the graded channel and the barrier layer on the HEMT properties is investigated and discussed. The optimized graded channel HEMT structure is found to have similarly high electron density (∼ 9 × 10 12 cm−2) as the non-graded conventional structure, though the mobility drops from ∼ 2360 cm2/V s in the conventional to ∼ 960 cm2/V s in the graded structure. The transconductance gm of the linearly graded channel HEMTs is shown to be flatter with smaller g m ′ and g m ″ as compared to the conventional non-graded channel HEMT implying improved device linearity.

Hot-Wall MOCVD for High-Quality Homoepitaxy of GaN: Understanding Nucleation and Design of Growth Strategies

Thick GaN layers with a low concentration of defects are the key to enable next-generation vertical power electronic devices. Here, we explore hot-wall metalorganic chemical vapor deposition (MOCVD) for the development of GaN homoepitaxy. We propose a new approach to grow high-quality homoepitaxial GaN in N2-rich carrier gas and at a higher supersaturation as compared to heteroepitaxy. We develop a low-temperature GaN as an optimum nucleation scheme based on the evolution and thermal stability of the GaN surface under different gas compositions and temperatures. Analysis in the framework of nucleation theory of homoepitaxial layers simultaneously grown on GaN templates on SiC and on hydride vapor phase epitaxy GaN substrates is presented. We show that residual strain and screw dislocation densities affect GaN nucleation and subsequent growth leading to distinctively different morphologies of GaN homoepitaxial layers grown on GaN templates and native substrates, respectively. The established comprehensive picture provides a guidance for designing strategies for growth conditions optimization in GaN homoepitaxy. GaN with atomically flat and smooth epilayer surfaces with a root-mean-square roughness value as low as 0.049 nm and low background carbon concentration of 5.3 × 1015 cm–3 has been achieved. It is also shown that there is no generation of additional dislocations during homoepitaxial growth. Thus, our results demonstrate the potential of the hot-wall MOCVD technique to deliver high-quality GaN material for vertical power devices.

GaN-based light-emitting materials prepared by hot-wall metal-organic chemical vapor deposition

GaN-based light-emitting materials prepared by hot-wall metal-organic chemical vapor deposition

Mg-doping and free-hole properties of hot-wall MOCVD GaN

The hot-wall metal-organic chemical vapor deposition (MOCVD), previously shown to enable superior III-nitride material quality and high performance devices, has been explored for Mg doping of GaN. We have investigated the Mg incorporation in a wide doping range (2.45×1018 cm−3 up to 1.10×1020 cm−3) and demonstrate GaN:Mg with low background impurity concentrations under optimized growth conditions. Dopant and impurity levels are discussed in view of Ga supersaturation, which provides a unified concept to explain the complexity of growth conditions impact on Mg acceptor incorporation and compensation. The results are analyzed in relation to the extended defects, revealed by scanning transmission electron microscopy, x-ray diffraction, and surface morphology, and in correlation with the electrical properties obtained by Hall effect and capacitance–voltage (C–V) measurements. This allows to establish a comprehensive picture of GaN:Mg growth by hot-wall MOCVD providing guidance for growth parameters optimization depending on the targeted application. We show that substantially lower H concentration as compared to Mg acceptors can be achieved in GaN:Mg without any in situ or post-growth annealing resulting in p-type conductivity in as-grown material. State-of-the-art p-GaN layers with a low resistivity and a high free-hole density (0.77 Ω cm and 8.4×1017 cm−3, respectively) are obtained after post-growth annealing demonstrating the viability of hot-wall MOCVD for growth of power electronic device structures.

Epitaxial growth of β-Ga2O3 by hot-wall MOCVD

The hot-wall metalorganic chemical vapor deposition (MOCVD) concept, previously shown to enable superior material quality and high performance devices based on wide bandgap semiconductors, such as Ga(Al)N and SiC, has been applied to the epitaxial growth of β-Ga2O3. Epitaxial β-Ga2O3 layers at high growth rates (above 1 μm/h), at low reagent flows, and at reduced growth temperatures (740 °C) are demonstrated. A high crystalline quality epitaxial material on a c-plane sapphire substrate is attained as corroborated by a combination of x-ray diffraction, high-resolution scanning transmission electron microscopy, and spectroscopic ellipsometry measurements. The hot-wall MOCVD process is transferred to homoepitaxy, and single-crystalline homoepitaxial β-Ga2O3 layers are demonstrated with a 2̄01 rocking curve width of 118 arc sec, which is comparable to those of the edge-defined film-fed grown (2̄01) β-Ga2O3 substrates, indicative of similar dislocation densities for epilayers and substrates. Hence, hot-wall MOCVD is proposed as a prospective growth method to be further explored for the fabrication of β-Ga2O3.

GaN-based pyramidal quantum structures for micro-size light-emitting diode applications

GaN-based pyramidal quantum structures, InGaN nanostructures located on top of micro-sized GaN pyramids, were fabricated by selective-area growth on SiC substrates by means of hot-wall metal-organic chemical vapor deposition. Arrays of GaN-based pyramidal structures exhibit micro-size pyramids possessing high uniformity, precise hexagonal bases, and InGaN/GaN quantum-well layers with well-defined interfaces. Each pyramid comprises a p-i-n InGaN/GaN structure, which is separated from that of other pyramids by a dielectric layer, serving as a building block for micro-emitters. Moreover, interconnected micro-size light-emitting diodes (microLEDs) built on the GaN-based pyramidal quantum structures were demonstrated, resulting in well-determined electroluminescence in the near-ultraviolet regime with negligible spectral shifts at high current levels. The results elucidated the rewards for development of these light-emitting designs and their potential for microLED applications.

(Invited) MOCVD of 2D Nanomaterials for Next-Generation Electronic and Optoelectronic Devices

The international road map of semiconductors (ITRS) lists 2D materials as possible future materials for electronic devices [ITRS]. Among them, the semiconducting transition metal dichalcogenides (TMDC) like MoS2 or WS2 are the most promising ones. As the miniaturization of electronic devices continues and the sub-5 nm gate limit is reached, direct source-to-drain tunneling will become a problem in Si devices with conventional architecture. 2D TMDC as channel material for FET might overcome this issue in digital CMOS. The larger carrier effective masses of most TMDC result in reduced direct source-to-drain tunneling, while they also yield a large density of states and hence an increased ballistic current in this extreme dimensions [Fiori]. Additionally, the lower in-plane dielectric constants of TMDC enable a better vertical electrostatic control over the channel [Desai]. Still, TMDC will have to compete against other approaches, e.g. multiple-gate transistors [Schwiertz]. Beyond that, other properties of TMDC turn them interesting for future devices, such as their strong absorbance across the visible spectrum for optoelectronic devices [Lotsch]. The first photodetectors, electromluminescent p-n diodes and photovoltaic cells have already been shown [Lopez, Cheng, Tsai]. The excellent mechanical properties additionally enable the realization of different kinds of flexible devices [Schwiertz]. And last but not least, the integration of 2D materials into conventional 3D semiconductor concepts would be another application. For example, the strain originating from the large difference in lattice constants in a 3D heterostructure is often a challenge. Inserting 2D materials might be a solution. Due to the fully terminated surface of a TMDC monolayer, the binding to another material is of van-der-Waals type. Hence, TMDC can be stacked with materials with huge differences in lattice constants without leading to considerable strain or to interface trap densities [Vogel]. First devices with a combination of 2D and 3D materials have already been investigated [Krishna, Lee]. Large-scale fabrication of TMDC is still a challenge to be overcome. Up to now, the most frequently used techniques are either exfoliation of natural layered crystals or the growth via chemical vapor deposition (CVD). Exfoliation is a very time-consuming process with relatively low yield and reproducibility. In CVD, different gaseous precursors decompose on a substrate and react with each other. Thermally evaporated MoO3 and S are commonly used as precursors for the fabrication of MoS2 monolayers via CVD. This technique allows the deposition of monolayered crystallites with a size in the range of micrometers [Dumcenco]. The process is carried out in small experimental reactors, and a uniform deposition on a whole wafer is very challenging. The use of metalorganic precursors is one possibility to enter an industrial scale of fabrication. The development of III/V and II/VI semiconductors has shown that metalorganic chemical vapor deposition (MOCVD) makes controllable and reproducible processes feasible, which are easily scalable and hence suitable for large-area deposition. Additionally, the reactors are well-developed, and a future integration of 2D materials and other semiconductors is within reach. First publications show very good results on the deposition of MoS2 and WSe2 via MOCVD [Kang, Eichfeld]. But despite these works and first simulations of the growth kinetics [Nie], little is known about the growth mechanism. For this reason, we have started to investigate the deposition of 2D MoS2 on an AIXTRON MOCVD reactor.The experiments are carried out in a horizontal hot-wall MOCVD reactor in a 10 × 2 inch configuration. Molybdenum hexacarbonyl (MCO) and Di-tert-butyl sulfide (DTBS) are used as Mo and S sources, respectively. In preliminary experiments, the carrier gas based transport of the precursors into the reactor and possible growth conditions are tested. Their results are used to develop an initial growth process. This process leads to a uniform, wafer-scale deposition of MoS2 on various substrate types such as sapphire (0001), Si (111), and AlN and GaN templates on sapphire substrates. The deposited films mainly consists of MoS2 bilayers and exhibit a very high initial nucleation density. Further investigations of the influence of the growth temperature, the carrier gas composition and the pretreatment of the substrates are carried out. With optimization of these growth parameters, a crystal growth process closer to thermodynamical equilibrium can be achieved. This results in the formation of triangular crystals on sapphire substrates which are also reported from CVD processes and exhibit higher crystal quality [Dumcenco]. Additional experiments are conducted to investigate the nucleation of the films and to further tune nucleation density and lateral growth rate in order to deposit wafer-scale monolayered MoS2 films.

Performance Enhancement of Microwave GaN HEMTs Without an AlN-Exclusion Layer Using an Optimized AlGaN/GaN Interface Growth Process

The impact of the sharpness of the AlGaN/GaN interface in high-electron mobility transistors (HEMTs) is investigated. Two structures, one with an optimized AlGaN/GaN interface and another with an unoptimized, were grown using hot-wall metal-organic chemical vapor deposition. The structure with optimized sharpness of the interface shows electron mobility of 1760 cm2/V $\cdot $ s as compared with 1660 cm2/V $\cdot $ s for the nonoptimized interface. Gated Hall measurements indicate that the sharper interface maintains higher mobility when the electrons are close to the interface compared with the nonoptimized structure, indicating less scattering due to alloy disorder and interface roughness. HEMTs were processed and evaluated. The higher mobility manifests as lower parasitic resistance yielding a better dc and high-frequency performance. A small-signal equivalent model is extracted. The results indicate a lower electron penetration into the buffer in the optimized sample. Pulsed- IV measurements imply that the sharper interface provides less dispersive effects at large drain biases. We speculate that the mobility enhancement seen AlGaN/AlN/GaN structures compared with the AlGaN/GaN case is not only related to the larger conduction band offset but also due to a more well-defined interface minimizing scattering due to alloy disorder and interface roughness.

Low thermal resistance of a GaN-on-SiC transistor structure with improved structural properties at the interface

The crystalline quality of AlGaN/GaN heterostructures was improved by optimization of surface pretreatment of the SiC substrate in a hot-wall metal-organic chemical vapor deposition reactor. X-ray photoelectron spectroscopy measurements revealed that oxygen- and carbon-related contaminants were still present on the SiC surface treated at 1200°C in H2 ambience, which hinders growth of thin AlN nucleation layers with high crystalline quality. As the H2 pretreatment temperature increased to 1240°C, the crystalline quality of the 105nm thick AlN nucleation layers in the studied series reached an optimal value in terms of full width at half-maximum of the rocking curves of the (002) and (105) peaks of 64 and 447arcsec, respectively. The improvement of the AlN growth also consequently facilitated a growth of the GaN buffer layers with high crystalline quality. The rocking curves of the GaN (002) and (102) peaks were thus improved from 209 and 276arcsec to 149 and 194arcsec, respectively. In addition to a correlation between the thermal resistance and the structural quality of an AlN nucleation layer, we found that the microstructural disorder of the SiC surface and the morphological defects of the AlN nucleation layers to be responsible for a substantial thermal resistance. Moreover, in order to decrease the thermal resistance in the GaN/SiC interfacial region, the thickness of the AlN nucleation layer was then reduced to 35nm, which was shown sufficient to grow AlGaN/GaN heterostructures with high crystalline quality. Finally, with the 35nm thick high-quality AlN nucleation layer a record low thermal boundary resistance of 1.3×10−8m2K/W, measured at an elevated temperature of 160°C, in a GaN-on-SiC transistor structure was achieved.

Dispersive Effects in Microwave AlGaN/AlN/GaN HEMTs With Carbon-Doped Buffer

Aluminium gallium nitride (AlGaN)/GaN high-electron mobility transistor performance is to a large extent affected by the buffer design, which, in this paper, is varied using different levels of carbon incorporation. Three epitaxial structures have been fabricated: 1) two with uniform carbon doping profile but different carbon concentration and 2) one with a stepped doping profile. The epitaxial structures have been grown on 4H-SiC using hot-wall metal–organic chemical vapor deposition with residual carbon doping. The leakage currents in OFF-state at 10 V drain voltage were in the same order of magnitude ( $10^{-4}$ A/mm) for the high-doped and stepped-doped buffer. The high-doped material had a current collapse (CC) of 78.8% compared with 16.1% for the stepped-doped material under dynamic $I$ – $V$ conditions. The low-doped material had low CC (5.2%) but poor buffer isolation. Trap characterization revealed that the high-doped material had two trap levels at 0.15 and 0.59 eV, and the low-doped material had one trap level at 0.59 eV.

Properties of GaN layers grown on N-face free-standing GaN substrates

GaN layers were homoepitaxially grown on N-face free-standing GaN substrates using a hot-wall metalorganic chemical vapor deposition method. By using optimized growth parameters, layers with a smooth morphology were obtained. The crystalline quality of epilayers was studied by a high resolution X-ray diffraction technique and compared to the substrates. Optical properties of the epilayers studied by low temperature time-resolved photoluminescence have shown longer recombination time for donor-bound exciton compared to the substrates.

Impact of residual carbon on two-dimensional electron gas properties in AlxGa1−xN/GaN heterostructure

High tuneability of residual carbon doping is developed in a hot-wall metalorganic chemical vapor deposition reactor. Two orders of temperature-tuned carbon concentration, from ∼2 × 1018 cm−3 down to ∼1 × 1016 cm−3, can be effectively controlled in the growth of the GaN buffer layer. Excellent uniformity of two-dimensional electron gas (2DEG) properties in AlxGa1−xN/AlN/GaN heterostructure with very high average carrier density and mobility, 1.1 × 1013 cm−2 and 2035 cm2/V·s, respectively, over 3" semi-insulating SiC substrate is realized with the temperature-tuned carbon doping scheme. Reduction of carbon concentration is evidenced as a key to achieve high 2DEG carrier density and mobility.

High-quality AlN layers grown by hot-wall MOCVD at reduced temperatures

We report on a growth of AlN at reduced temperatures of 1100 °C and 1200 °C in a horizontal-tube hot-wall metalorganic chemical vapor deposition reactor configured for operation at temperatures of up to 1500–1600 °C and using a joint delivery of precursors. We present a simple route—as viewed in the context of the elaborate multilayer growth approaches with pulsed ammonia supply—for the AlN growth process on SiC substrates at the reduced temperature of 1200 °C. The established growth conditions in conjunction with the particular in-situ intervening SiC substrate treatment are considered pertinent to the accomplishment of crystalline, relatively thin, ∼700 nm, single AlN layers of high-quality. The feedback is obtained from surface morphology, cathodoluminescence and secondary ion mass spectrometry characterization.

Growth of Ruthenium and Ruthenium oxide nanoplates

ABSTRACTBy carefully manipulating and controlling the growth conditions, Ruthenium (Ru) and ruthenium oxide (RuO2) two-dimensional (2-D) nanostructure were self-assembled into a stack of plates on indium tin oxide coated glass substrate. The nanoplates were grown in a horizontal hot-wall metalorganic chemical vapor deposition (MOCVD) from ruthenocene. Each nanoplate has a thickness in the range of 25 - 60 nm and the average area is 1000 x 300 nm2. Each stack of nanoplates is approximately 1.2 m in height. A continuous layer of Ru and RuO2 thin film, which may serve as the growth template, is observed on the bottom of the nanoplate stacks. Field-emission scanning electron microscopy reveals that each stack of nanoplates was grown vertically aligned on the substrate and exhibited elongated shape. Structural properties which were examined by X-ray diffraction show that the nanoplates are polycrystalline.

Improved hot-wall MOCVD growth of highly uniform AlGaN/GaN/HEMT structures

The inherent advantages of the hot-wall metal organic chemical vapor deposition (MOCVD) reactor (low temperature gradients, less bowing of the wafer during growth, efficient precursor cracking) compared to a cold-wall reactor make it easier to obtain uniform growth. However, arcing may occur in the growth chamber during growth, which deteriorates the properties of the grown material. By inserting insulating pyrolytic BN (PBN) stripes in the growth chamber we have completely eliminated this problem. Using this novel approach we have grown highly uniform, advanced high electron mobility transistor (HEMT) structures on 4″ semi-insulating (SI) SiC substrates with gas-foil rotation of the substrate. The nonuniformities of sheet resistance and epilayer thickness are typically less than 3% over the wafer. The room temperature hall mobility of the 2DEG is well above 2000 cm 2/V s and the sheet resistance about 270 Ω/sqr.

Uniform hot-wall MOCVD epitaxial growth of 2 inch AlGaN/GaN HEMT structures

The hot-wall metalorganic chemical vapor deposition (MOCVD) concept has been applied to the growth of Al x Ga 1− x N/GaN high electron mobility transistor (HEMT) device heterostructures on 2 inch 4H-SiC wafers. Due to the small vertical and horizontal temperature gradients inherent to the hot-wall MOCVD concept the variations of all properties of a typical HEMT heterostructure are very small over the wafer: GaN buffer layer thickness of 1.83 μm±1%, Al content of the Al x Ga 1− x N barrier of 27.7±0.1%, Al x Ga 1− x N barrier thickness of 25 nm±4%, sheet carrier density of 1.05×10 13 cm −2±4%, pinch-off voltage of −5.3 V±3%, and sheet resistance of 449 Ω±1%.

Superior material properties of AlN on vicinal 4H-SiC

The crystal structure and optical properties of thick (>100nm) AlN layers grown by hot-wall metalorganic chemical vapor deposition are characterized by infrared spectroscopic ellipsometry, cathodoluminescence, and transmission electron microscopy. The choice of substrates among the available SiC wafer polytype modifications (4H∕6H) and misorientations (on-/off-axis cut) is found to determine the AlN defect interaction, stress homogeneity, and luminescence. The growth of thick AlN layers benefits by performing the epitaxy on off-axis substrates because, due to stacking faults, the propagation of threading defects in AlN layers is stopped in a narrow interface region.